This invention generally relates to purification of uranium enriched solutions. The in-situ leaching of mineral values from subterranean deposits is well known in the art as a practical means for recovering certain elements such as uranium, molybdenum, vanadium and the like. Basically, solution mining is carried out by injecting into the subterranean deposit, a lixiviant solution which will solubilize the mineral value desired to be recovered. The solution and solubilized mineral values are then recovered from the deposit for subsequent separation of the mineral values. Often it is necessary to oxidize the mineral value to a form where it can form a soluble reaction product in the lixiviant solution.
Depending upon the nature of the subterranean deposit, the typical lixiviant solution may be an acid, for example, an aqueous sulfuric acid solution, or may comprise an alkaline carbonate solution. In view of the high proportion of carbonates typically present in some subterranean formations, the use of acid solution in these ore bodies is usually prohibitive, because of the excessive consumption of acid due to carbonate solubilization. Consequently, alkaline carbonate lixiviant solutions are preferred to acid solutions for solution mining operations when high levels of carbonates are present in the formation. Various solution mining processes, involving the use of alkaline carbonate and non-alkaline carbonate leaching solutions are taught in U.S. Pat. No. 2,992,887 and U.S. Pat. No. 4,105,253 respectively.
The lixiviant, enriched with uranium, along with other ions such as calcium, molybdenum and vanadium, and other trace species, is pumped to the surface, where the uranium is recovered, generally by some type of ion exchange process. During the ion exchange process, the uranium is exchanged for other anions. Typically, the preferred anion is chloride, but other anions can be used, such as sulfate and carbonate. A similar approach using hollow fiber, flat sheet or tubular shaped polymeric ion exchange membranes, in a membrane extraction process, is taught by Ho et al., in U.S. Pat. No. 3,957,504.
After the ion exchange process, the barren lixiviant is refortified with bicarbonate, and oxidant and is recycled through the underground formation. In this manner, as the uranium is leached from the formation, the anion exchanged for the uranium increases in concentration in the lixiviant. At the end of the mining phase, the lixiviant must be returned to its initial groundwater state, i.e., all ions in solution must be near their initial or some predetermined, acceptable concentration. As such, the chloride or other anions that were exchanged for uranium, as well as residual lixiviant constituents, must be removed from the water at considerable expense.
Reverse osmosis processes are well known as means for purifying sea or waste water solutions, containing from about 100 to 5,000 parts, per million parts water, of salt or other dissolved solids, as taught by Stana, in U.S. Pat. No. 3,593,855. Reverse osmosis has also been used to remove dissolved minerals, such as iron, calcium, magnesium, manganese, and aluminum from sulfate containing, contaminated, acid mine drain waters, which have generally been pretreated by chlorination and ferrous iron oxidation, as taught by Hill et al., in U.S. Pat. No. 3,795,609.
Sastri and Ashbrook, in Separation Science, 11(4), pp. 361-376, 1961, describe the use of single step reverse osmosis as the means to remove uranyl sulfate, UO.sub.2 SO.sub.4, from mine water feed. The metal ions separated are Ca.sup.+2, Fe.sup.+3, Al.sup.+3 and U.sup.+6, using supported, preshrunk, "tight", cellulose acetate permselective membranes, having rejection rates of between about 50% to 90% on aqueous NaCl. Such a system would have a very low purified material flux.
What is needed, is a commercially feasible, economical process, specifically adapted to concentrate the small amounts of uranium in the natural valence state, present in enriched lixiviant solutions, while maintaining the basic lixiviant solution for recycle to the underground formation. This process should limit the requirement of addition of any chemicals or introduction of any ions that will interfere with the further processing of the uranium rich concentrate or contaminate the lixiviant that is returned to the subterranean deposit.